1. Technical Field
This invention relates to an improved electrode for electrolysis and a process for its production.
2. Description of the Prior Art
Heretofore, most of the alkali metal salt electrolysis represented by the electrolysis of saline water has been done by the so-called "mercury process". In recent years, however, the environmental pollution caused by mercury contained in the waste matter from the process has become a serious social problem, and a turnabout from the "mercury process" to the so-called "diaphragm process" and "ion-exchange membrane process" has been required.
Usually, in the diaphragm process and the ion-exchange membrane process, a higher pH value than in the mercury process is used at the time of the electrolysis, and the electrodes which are known are generally low in their oxygen overpotential. On account of this, the asbestos diaphragm process and the ion-exchange membrane process necessarily mixes 1 to 3% or so of oxygen into chlorine gas to be generated. As the consequence of this, the chlorine obtained from these processes cannot be supplied directly to petro-chemical plants, rather the oxygen must be removed prior to its use. On account of this, special apparatus and complicated operations are necessary, which constitutes one of the causes for increased cost of production.
In solving such a problem an electrode which generates as low an amount of oxygen as possible may be used. However, the equilibrium potential EO.sub.2 of oxygen is lower than the equilibrium potential ECl.sub.2 of chlorine, owing to which an electrode having no selectivity at all with respect to the electrode reaction with oxygen and chlorine should always accompany generation of a large amount of oxygen at a potential for the chlorine generation.
In order therefore to suppress generation of oxygen as far as possible, it is necessary that the shielding material for the electrode be imparted with a characteristic which tends to make it difficult for the oxygen electrode reaction proceed from the standpoint of the theory of rate process.
Usually, the selectivity of the electrode to such reaction is termed "electrocatalysis", wherein the exchange current density of each shielding material for the electrode is used as the yardstick. Various platinum group elements such as ruthenium, palladium, rhodium, platinum, and iridium are examples of elements which exhibit electrocatalysis.
When these elements are arranged in the order of magnitude of their exchange current density, they are Ru&gt;Ir&gt;Rh&gt;Pd&gt;Pt in respect of the oxygen electrode reaction, while they are Pd&gt;Ru&gt;Ir&gt;Rh&gt;Pt in respect of the chlorine electrode reaction.
From the above, it is seen that palladium best serves the purpose in the point that it produces less oxygen and is excellent in its electrocatalysis for the chlorine electrode reaction.
In practical use, however, when the electrode is coated with palladium in metal form, its corrosion-resistant property is inferior and the coating becomes dissolved at the time of the electrolysis with the consequent problem of its inability of being put to practical use.
With a view to solving this problem, there have been proposed various corrosion-resistant electrodes such as that made of a platinum/palladium alloy, or that made by coating a substrate (base plate) with this alloy, or that obtained by oxidizing the surface of this alloy (vide: Japanese patent publication No. 11014/1970, Japanese patent publication No. 11015/1970). These electrodes, however, do not exhibit the excellent electrocatalysis of palladium per se, as the palladium has been changed to an alloy, and, moreover, are not durable with respect to their corrosion-resistant property over a long period of time.
There has also been proposed an electrode made of an oxide of platinum/palladium alloy (vide: Japanese patent publication No. 3954/1973). In practice, however, the alloy oxide must be treated in an oxygen atmosphere at a high temperature and under a high pressure for the alloy oxide to be formed on a titanium base plate. In so doing, the titanium base plate undergoes considerable oxidation and is incapable of being used as the electrode. On account of this, the abovedescribed method coats the platinum/palladium alloy on the titanium base plate to form the alloy oxide by anodic oxidation, although it is similar to as the above mentioned electrode of the alloy, the surface of which has been oxidized.
On the other hand, an attempt has also been made to coat palladium in its oxide form on a base plate of a valve metal such as titanium, by first applying a compound which turns into palladium oxide through pyrolysis on the base plate, and then thermally decomposing the same. However, no success could be attained because of low adhesion between the titanium base plate and palladium oxide.
After much investigation, it was found that improved adhesive property is obtained when a small quantity of other metal oxide is included in a large quantity of palladium oxide for the coating at the time of effecting the pyrolytic process. However, when the palladium oxide coating is to be applied on the titanium base plate by the pyrolytic process, if titanium is in direct contact with palladium oxide or unreacted palladium compound as the raw material, the palladium compounds are reduced by titanium to deposit metallic palladium, which then enters the palladium oxide as a mixture component. Due to this, even when the adhesive property becomes improved to a certain degree, the electrode so formed is of no practical use, since the metallic palladium which has been deposited as mentioned above dissolves at the time of the electrolysis to render the coating to be porous, which tends to cause the coating to be separated from the base plate with generation of gas from the electrode surface, changing its corrosion-resistant property with lapse of time.
In view of these facts, the present inventors have previously proposed a process for producing an electrode, wherein a coating consisting of palladium oxide as a perfect oxide and a platinum metal is applied on the valve metal base plate made of titanium, tantalum, zirconium, and so forth (vide: Japanese patent publication No. 8595/1980, and others).
The feature of this process for producing the electrode is not to apply directly the palladium compound capable of being pyrolyzed on the base material, followed by the thermal decomposition, but is to pyrolyze, beforehand, palladium chloride, for example, in the oxygen atmosphere, or to oxidize, in advance, palladium black in the oxygen atmosphere to thereby prepare perfect palladium oxide.
The thus prepared palladium oxide is dispersed in a butanol solution of a platinum compound which turns into the platinum metal by pyrolysis such as, for example, chloroplatinic acid, with addition of a dispersing agent, to thereby prepare a coating liquid. This coating liquid is applied onto the base plate which has been subjected to mechanical and chemical etching, followed by baking. According to this method, production of the metallic palladium cannot be recognized at all, and, moreover, a film thickness several times as thick as that obtained by the conventional thermal decomposition process can be obtained by one coating operation, due to which the corrosion-resistant property of the electrode improves. It is noted that, platinum to be contained simultaneously with palladium, in this case, is required to be platinum metal in its coated condition.
This improves the adhesive property between palladium oxide and the base plate, on which palladium oxide is to be coated, and improves the electrical contact among particles of palladium oxide so as to cause them to decrease the electrical resistance of palladium oxide which facilitates the electro-chemical catalysis.
While the above mentioned electrode is satisfactory as to its use in electrocatalysis and corrosion-resistant property, it has serious the defect that the electrode coating tends to bring about mechanical separation with generation of foams at the time of the electrolysis.
In order to solve this shortcoming, methods have already been proposed wherein, after the above mentioned electrode coating has been formed, a platinum metal coating is further applied thereon at a later stage from a compound which turns into the platinum metal by this pyrolysis, and a method, in which the electrode coating and the platinum metal coating to be formed by the above mentioned method are applied in multi-layered structure in an arbitrary sequence (vide: Japanese unexamined patent publication No. 43879/1979 and Japanese patent publication No. 36713/1980). According to these methods, the electrocatalysis and the corrosion-resistant property of the resulting electrode are high, and moreover, the mechanical separation of the coating with generation of foams at the time of the electrolysis is remarkably reduced.
Nevertheless, such multi-layered coating with two liquid type coating liquid has such disadvantages that the number of its coating steps increases, its manufacturing becomes complicated and troublesome, and the film thickness or the coating as well as the quantitative ratio of the coating components are difficult to be controlled.
Further, the electrode for electrolysis is often subject to friction and/or scratching from various objects such as supporting mechanisms for tools, packing materials, human hands, diaphragms, and others during the working processes after formation of the coating film, during transportation of the electrode after the working, or during its installation into the electrolytic vessel, or further during the electrolytic operation. In such circumstances, the electrode as mentioned above is insufficient in its mechanical strength against such friction, etc., even if the mechanical separation of the coating due to generation of foams is reduced.
In addition, a fairly large amount of platinum which is not abundant in the natural resources is used.
In the proposed method of multi-layered coating as mentioned in the foregoing, it is also noted that, when cerium, zirconium, titanium, tantalum, tungsten, or other metals are simultaneously added to the coating liquid in the form of halides, organic salts, and so on, and are turned into oxides by heating, and included in the coating, the mechanical strength of the coating is improved. It is certain that, when these oxides are included, the mechanical strength of the coating is improved even against the above mentioned friction, but the improvement is still not necessarily satisfactory, and various inconveniences have been recognized. For example, even when the mechanical strength to friction is satisfactory, the film resistance increases in that case to bring about a increase in the vessel voltage due to the increased chlorine overpotential, and the service life of the electrode becomes short.